The vault derives its strength from its shape and structure. The two tables are of equal strength, and, for the most part, separated from one another by a variable amount of diploic tissue. This diploe is most abundant in the frontal, parietal, and upper occipital regions. These parts are proportionately strong. Two regions are practically devoid of this inter-tabular buffer—the squamo-temporal and cerebellar (see [Figs. 29] and [30]). A recognition of this comparative weakness is of great practical importance in view of the fact that both these regions are liable to special lesions—injury to the middle meningeal artery in the first case, and, in the second, cerebello-medullary lesions. Nature’s ‘mistake’ in providing coverings unsuited to requirements has been compensated for in part by additional protection—the temporal and nuchal muscles.
Fig. 28. Diagram illustrating the Lines along which Forces received on the Vault are transmitted to the Base. (For further description, see text.)
Further, not only does the skull vary in density in its several parts, but it is also ribbed and strengthened by various bony bars and buttresses that pass up from base to vault (see [Fig. 28]). These ‘ribbings’ are seen to extend upwards from the crista galli, from the external angular frontal process, from the auditory region, and from the occipital protuberance. Presumably, these ‘ribbings’ were so constituted for a definite purpose; in any case, it is clear that they play an important part in the reception and conduction of forces to the base of the skull. It is apparent, moreover, that the parts intervening between these ‘ribbings’ are liable to injury in direct proportion to their general position and strength. The deep groovings of the bone for the reception of the middle meningeal artery afford an additional source of weakness to the bone in the squamo-temporal region. (See [Fig. 50]).
Fig. 29 a. The Base of the Skull.
Fig. 29 b. The Base of the Skull as seen on Transillumination.
Further reference will be made to the relative strengths of the various regions of the skull. Sufficient has been said to show that nature has provided the skull with various paths by means of which forces applied to the vault can be conducted and distributed to the base.
Before, however, proceeding to discuss the effects produced on the base of the skull, it is necessary to add that nature provides other methods by means of which the intensity of a blow, delivered over the vertex of the skull, is diminished. The forces are broken up and distributed in the following manner:—